Well tool devices used in oil and/or gas wells, such as different types of plugging devices (bridge plugs, packers etc) typically comprises a sealing element provided circumferentially around a mandrel device. On each side (i.e. on the upper side and on the lower side) of the sealing element, supporting devices are provided. The well tool device may be configured to be in a run state (a radially retracted state) and a set state (a radially expanded state). The run state is used when running the well tool device into the well. In the set state, a relative axial movement between the supporting devices is causing the sealing element to be compressed axially and hence to expand radially until the outer surface of the sealing element is in contact with the inner surface of the well pipe. Hence, a seal is provided in the annular space between the inner surface of the well pipe and the mandrel device and fluid flow between the lower side of the seal and the upper side of the seal is prevented.
In the set state, the well tool device is designed to withstand a pressure difference between the lower side of the sealing element and the upper side of the sealing element.
The well tool device typically also comprises a ratchet mechanism in order to allow axial movement of at least one of the supporting device in one (a forward) direction (i.e. to move the sealing element from the run state to the set state) but to prevent movement of the at least one supporting device in the opposite (a reverse) direction (i.e. to prevent radial retraction of the sealing element).
There are some disadvantages with these prior art well tool devices.
First, if the well tool device is set in the well at a high temperature, a decrease in temperature may cause the sealing element to shrink. In such a situation, there is a risk that the sealing element may not withstand the same pressure difference as before the temperature decrease.
Second, when the well tool device has been set, the pressure below the sealing element will often increase to a pressure being higher than the pressure above the sealing element, since fluid flow is prevented by the sealing element. This pressure difference will apply a force to the well tool device which is contributing to a compression of the sealing element via the supporting devices. However, if the fluid pressure changes, for example if the pressure above the sealing element gets equal to, or higher than the pressure below the sealing element, there is a risk that the compression force applied by the supporting device onto the sealing element will be reduced.
Third, the ratchet mechanism will allow some movement also in the reverse direction, typically a length up to one teeth length. This reverse movement is often referred to as the “backlash” of the ratchet mechanism.
Hence, in these situations, there is a risk that the sealing element will not be in sufficient contact with the inner surface of the well pipe anymore. Consequently, the well tool device is no longer working as intended.
The above situations may occur in particular in injection wells, where the well tool is cooled down by the injected fluid and where the differential pressure over the sealing element will change before, during and after the injection operation, for example when a safety valve is maintained or tested, etc. Also well tools used in acidizing operations, fracking operations etc may be subjected to the abovementioned temperature/pressure conditions.
In prior art, these effects may be at least partially compensated for by means of relatively long springs (spiral springs or cup springs) applying an axial force onto the supporting devices towards the sealing element. The disadvantage with this solution is that it does not always work sufficiently. Moreover, this solution will increase the length of the well tool device considerably.
Moreover, as coiled springs only give a relatively small force, cup springs must be used in many such tools. The cup springs must often be hardened to achieve satisfying material properties, and if the well tool is to be NACE approved (National Association of Corrosion Engineers), only a few materials can be used, such as UNS N07750, UNS R30003, and UNS R30035. These materials are very expensive. For some applications, the well tool device will need a cup spring with a total length of 150-200 mm and will comprise ca 30 cups/discs.